Importance of basement faulting and salt decoupling for the structural evolution of the Fars Arc, Zagros fold-and-thrust belt: A numerical modeling approach

Understanding the tectonic evolution and crustal-scale structure of fold-thrust belts is crucial for exploring geological resources and evaluating seismic hazards. We conducted a series of finite-difference two-dimensional thermo-mechanical numerical models with visco-elasto-plastic/brittle rheology to decipher how the interaction of inherited basement faults and salt décollement levels control the deformation process and structural style of the Fars Arc in the Zagros fold -thrust belt, during tectonic inversion. Results indicate that initial rifting is controlled by the geometry of inherited faults. During the convergence phase, fold-and-thrust belts display folding at two scales: large wavelength folds induced by basement deformation in the form of fault-propagation faults, and small wavelength folds and thrust systems emerge above the salt layer as detachment folds. Reactivated faults can serve as pathways for stress transfer, resulting in the emergence of new faults and thus seismic activity. The tectonic events in orogenic belts like the Zagros do not adhere to a fixed pattern; they are shaped by factors such as the properties of basement rocks and the orientation of faults. Shallow earthquakes predominantly occur along décollement anticlines, while deeper and larger ones are associated with basement faults. Additionally, we observe variations in resistance to deformation based on salt rheology and fault geometry, with listric faults minimizing resistance. The degree of basement involvement in deformation directly influences the model's resistance, with greater involvement facilitating easier deformation. Our results showing the temporal-spatial relationship between thin- and thick-skinned tectonics can work as an analogue for similar orogenic belts worldwide, such as Taiwan, the Pyrenees, the Alps, the Appalachians, and the Kopet Dagh

Effect of fluid pressure distribution on the structural evolution of accretionary wedges

Numerical experiments on evolving accretionary wedges usually implement predefined weak basal décollements and constant strength parameters for overlying compressed sequences, although fluid pressure ratio, and therefore brittle strength, can vary strongly in sedimentary basins. A two-dimensional finite difference model with a visco-elasto-plastic rheology is used to investigate the influence of different simplified fluid pressure ratio distributions on the structural evolution of accretionary wedge systems. Results show that a linear increase in fluid pressure ratio towards the base leads to toeward-verging thrust sheets and underplating of strata, while simulations with a predefined décollement form conjugate shear zones supporting box-fold-type frontal accretion. Surface tapers are in agreement with the critical wedge theory, which here is modified for cases of varying fluid pressure ratio. Furthermore, the numerical results resemble findings from natural examples of accretionary wedges. © 2017 John Wiley & Sons Ltd.Financial support was provided by the Swiss National Science Foundation (grant 2-77297-15).Peer reviewe

Effects of fault-weakening processes on oblique intracontinental rifting and subsequent tectonic inversion

In many mountain belts, deformation concentrates along mechanically weak fault zones inherited from earlier tectonic events. This work investigates the effects of two modes of structural weakening on the orientation of rifting and later tectonic inversion with respect to the imposed divergence/convergence direction in a high-resolution 3D finite difference model with a viscous-frictional rheology. In the first set of experiments, weakening consists in a decrease in frictional strength with increasing shear strain. The generated normal faults strike orthogonal to the imposed divergence direction. These faults are reactivated during tectonic inversion and absorb 50 to 70 percent of accumulated strain. In the second set of experiments, frictional strength is a decreasing function of shear strain rate. The generated faults are oblique to the divergence direction, implying oblique fault slip. Fault reactivation depends on the obliquity of the inverted rift to convergence direction, where larger obliquity leads to more intense fault reactivation. These new numerical results are compared to previous analogue and numerical models on the one hand, and natural examples of intracontinental mountain ranges due to tectonic inversion on the other hand. These comparisons demonstrate that both modes of frictional weakening should be taken into account when seeking to understand large-scale rifting and inversion tectonics.Financial support was provided by the Swiss National Science Foundation (grant 2-77297-15).Peer reviewe

Numerical modelling of inversion tectonics in fold-and-thrust belts

This work presents numerical experiments of inversion of rift basins and consequent sub-thrust imbrication in tectonic wedges. Half-graben basins initially develop and then are covered with a post-rift sequence bearing a décollement-prone horizon (i.e., the upper décollement). A total of twelve models of tectonic inversion have been conducted varying (i)the strength of inherited extensional fault arrays and (ii)applying different fluid pressure ratios (i.e., strength)within syn-rift strata. Combinations of those were simulated using different internal angles of friction for the inherited faults, different strengths for the syn-rift infill and for the upper décollement. Results show that changes in relative strength between inherited faults, syn-rift deposits and the upper crustal décollement leads to important variations in structural styles. Weak faults systematically favour the compressional reactivation of inherited extensional faults. Weak syn-rift sediments favour hanging wall by-pass structures instead of fault reactivation and less internal deformation of the syn-rift deposits. Weak upper décollements supports the accretion of basement in a hinterland antiformal stack, decoupling of basement and cover, and forward tectonic transport of rift basins. Strong upper crustal décollements favours basement and cover coupling, can lead to fault reactivation in the absence of weak faults and syn-rift sediments, however combinations of weak faults and strong upper décollement shows fault reactivation, weak syn-rift sediments and strong upper décollement form hanging wall by-pass structures. Modelling results are compared to natural case studies. © 2019This is a contribution of the Institut de Recerca Geomodels and the Geodinàmica i Analisi de Conques research group (2014SGR467SGR). Jonas B. Ruh was supported by the Swiss National Science Foundation grant number 2-77297-15 .Peer reviewe

Subduction and accretion of the Briançonnais continental slice: its effect on the Alpine orogenic wedge

Peer Reviewe

Finite-difference numerical modeling of sub-thrust inversion tectonics

In collisional foredeeps, rifted continental margins become buried by foreland sedimentary deposits and, ultimately, are overridden by advancing fold-and-thrust belts. With ongoing collision, stresses are transmitted into the foreland plate, causing positive inversion of buried rift basins in the sub-thrust region; ultimately, rift basins may become incorporated into the thrust system. This process indicates the co-existence of two crustal detachments soled at different depths within forelands: i) a shallow level décollement decoupling molasse-type sequences from the underlying basement and ii) deep-rooted lower crustal shear zones causing thick-skinned deformation. The activity of those detachment horizons is governed by different environmental conditions and deformation mechanisms. As today, dynamic strain weakening of brittle faults and fluid overpressures are recognized as fundamental parameters controlling fault reactivation necessary for basin inversion. In natural systems, however, these two factors have to compete against the lithostatic load imposed by the advancing fold-and-thrust belt. To test the relative influence and potential feedbacks of these parameters for the reactivation and inversion of rift basins beneath fold-and-thrust belts, we have carried out a series of numerical simulations applying a fully-vectorised 2D finite-difference code with marker-in-cell technique and a visco-elasto-plastic/brittle rheology. The two main parameters investigated are the strength of high-strained fault zones and the fluid overpressure of the syn-rift sequences. Results indicate that the structural evolution of the fold-and-thrust belt is affected by the competition between the strength of weakened faults and that of syn-rift strata. Relatively weak inherited fault zones lead to sub-thrust basin inversion by fault reactivation, whereas highly overpressured syn-rift strata favour the formation of hanging-wall bypass thrusts rather than fault reactivation

Effects of reactivated extensional basement faults on structural evolution of fold-and-thrust belts: Insights from numerical modelling applied to the Kopet Dagh Mountains

The structural evolution of basement-involved, or thick-skinned, fold-and-thrust belts is often affected by preexisting, inherited extensional faults within the basement. Here, a crustal-scale two-dimensional finite difference model with a visco-elasto-brittle/plastic rheology is applied to investigate the formation of fold-and-thrust belts as a result of tectonic inversion, from intracontinental extension to compression. We examine the influence of frictional strain weakening, varying surface process intensity and different crustal rheologies. Intense strain weakening results in narrow and deep basins with wide shear zone spacing during the extension phase and fault reactivation and localized uplift during inversion. Little weakening of shear zones results in thrusts cross-cutting pre-existing normal faults and the development of newly-formed thrusts in distal parts of the fold-and-thrust belt during inversion. Enhanced surface mass movement localizes deformation and uplift in the central part of mountain belts. Implementation of a weak upper crust (quartzite) leads to less localized deformation compared to a strong crust (quartz-diorite). We compare modelling results to the basement-involved Kopet Dagh Mountains, NE Iran, and discuss potential lateral variation of fault reactivation and erosion rates being responsible for deeper tectonic exhumation in the eastern part of the mountain belt, relative to the West and Central Kopet Dagh. © 2017 Elsevier B.V.Financial support was provided by the Swiss National Science Foundation (grant 2-77297-15).Peer reviewe

Hybrid thrust sequences - A new structural perspective

21 pages, 15 figures.-- Data availability: Data will be made available on requestThrust sequences show a range of geometric and kinematic characteristics in cross-sections and in map view. Two types of thrust sequences are commonly observed in thrust systems – forward-breaking and break-back thrust sequences. For nearly half a century, it has been hypothesized that forward-breaking and break-back thrust sequences represent end-members of a continuous spectrum of brittle and ductile structures. Many natural and experimental observations of transitional thrust sequences display both forward-breaking and break-back (hybrid) thrust sequences within the same thrust system. An explanation of hybrid thrust sequence development has neither been provided conceptually nor experimentally. We present geometric and kinematic results of natural observations and numerical experiments that show a transition from forward-breaking to break-back thrust sequences caused by the effects of stratigraphic variations, sedimentary environment, surface processes, critical-wedge effects, progressive deformation with an increase in compressive stress, presence of detachments and their frictional properties, and strain rate. We also discuss numerous geometric, kinematic, chronological and mechanical criteria to identify the characteristic signatures of hybrid thrust sequences. This study provides a template to distinguish different thrust sequences and to characterize the deformation conditions under which hybrid thrust sequences develop in contractional settings. Mechanical and field criteria, displacement patterns imaged on seismic reflection sections across onshore and offshore thrust systems, and the way that thrusts interact in terms of geometry, kinematics and strain, are used to illustrate thrust developmentWith the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)Peer reviewe

Structural evolution of an inverted salt-related rift basin: 2D numerical modelling applied to the High Atlas, Morocco.

GEOMOD 2018 Barcelona, 1-4/10 2018The Central High Atlas in Morocco results from Cenozoic tectonic inversion of an inherited salt-related rift basin that developed during Late Triassic to Early Jurassic times, forming an intricate polygonal network of diapir walls separating minibasins (Moragas et al., 2017). In the present work, we conduct numerical models to test tectonic evolution of the Central High Atlas rift basin as proposed in the work mentioned above, from the onset of extension to compression, i.e., positive tectonic inversion. In particular, we analyse the key parameters controlling the dynamics of salt tectonics in an inverted fold-and-thrust belt, such as rheology and the spatial distribution of salt rock